The sequence node corresponds to a logical AND: it succeeds if all children succeed and fails if one child fails. The selector, also called fallback node, represents a logical OR:
If one child succeeds, the remaining ones will not be ticked. It fails only if all children fail. The parallel control flow node forwards ticks to all children and fails if one fails. A decorator allows to define custom functions. Implementations like extended Behavior Trees (eBT) in SkiROS [1]} add custom processors such as parallel-first-success that succeeds if one of the parallel running children succeeds.
Leaves of the BT are the execution nodes that, when ticked, execute one cycle and output one of the three signals: success, failure or running. In particular, execution nodes subdivide into 1) action and 2) condition nodes. An action performs its operation iteratively at every tick, returning running while it is not done, and success or failure otherwise. A condition performs an instantaneous operation and returns always success or failure and never running.
An example of the BT for the peg insertion task is in Fig. REF .
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